However, that's down from 13 percent of daily calories from 2003 to 2006, according to report author Cheryl Fryar.

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Fryar told U.S. News and World Report that it's unclear whether this is a trend in the right direction, partly because as people age, they tend to eat less fast food. Adults aged 60 and over consumed the lowest percentage of their daily calories from fast food -- just 6 percent.

"The good news from this study is that as we get older, perhaps we do get wiser and eat less fast food," Samantha Heller, a clinical nutritionist at the NYU Center for Musculoskeletal Care in New York City, told U.S. News. "However, a take-home message is that the study suggests that the more fast food you eat, the fatter you get."

Nationwide survey participants were asked to recall all of the food they'd eaten in the past 24 hours. "Restaurant fast food/pizza" was one choice.

Besides age, ethnic differences also emerged in the results, with the percent of calories from fast food higher among non-Hispanic black adults than among Hispanics and whites. According to the CDC report, among young non-Hispanic black adults, more than one-fifth of their calories were consumed from fast food.

Sex, however, was not a factor. The percentage of calories consumed from fast food did not differ significantly between men (11.8 percent) and women (10.9 percent).

Something that was no surprise: the more a person weighed, the more fast food they ate.

Highlights of the CDC report include:

• From 2007-2010, American adults got 11.3 percent of their daily calories from fast food.

• For those aged 20 to 39, fast food accounted for more than 15 percent of daily calories.

• For those 60 and older, fast food accounted for about 6 percent of daily calories.

• Young adult whites and Hispanics (aged 20 to 39) got about 15 percent of their daily calories from fast food.

• Blacks in that same age group got about 21 percent of their daily calories from fast food.